Impedance matching device and impedance matching method

ABSTRACT

An impedance matching device comprises a variable capacitor including multiple capacitance elements and connected in parallel having capacitors and each having one end connected in series to a high-frequency power source and semiconductor switches and connected to the respective capacitors, and a control unit. The control unit derives a reflection coefficient based on the obtained information concerning an impedance viewed from the high-frequency power source toward the load side, updates the states of the respective semiconductor switches and included in the multiple capacitance elements and with a first cycle in the case where the reflection coefficient is equal to or more than a predetermined value, and updates the states of the respective semiconductor switches and included in the plurality of capacitance elements and with a second cycle longer than the first cycle in the case where the reflection coefficient is less than the predetermined value.

CROSS-REFERENCE TO RELATED APPLICATION

This Non-provisional application claims priority under 35 U.S.C § 119(a) on Patent Application No. 2018-243192 filed in Japan on Dec. 26, 2018, the entire contents of which are hereby incorporated by reference.

FILED

The present disclosure relates to an impedance matching device and an impedance matching method that achieve impedance matching between a high-frequency power source and a load.

BACKGROUND

An impedance matching device has been known that matches an output impedance of a high-frequency power source to an impedance viewed from the high-frequency power source toward a load side in order to eliminate reflected power from a load and efficiently supply power to the load when power is supplied from the high-frequency power source to the load such as a plasma load or the like (for example, refer to Japanese Patent application Laid-Open No. 2012-142285 as Patent Document 1).

The impedance matching device disclosed in Patent Document 1 includes a variable capacitor, and is provided between a high-frequency power source and a load. The variable capacitor disclosed in Patent Document 1 includes multiple capacitors connected in parallel to each other, and is configured to change the capacitance of the variable capacitor by performing switching control of turning on or off semiconductor switches (PIN diodes) connected in series to the respective capacitors. The impedance matching device disclosed in Patent Document 1 achieves impedance matching by changing the capacitance of the variable capacitor.

SUMMARY

However, in the case where high-frequency power is supplied to a load whose impedance frequently varies such as a plasma load, continuous updating of the states of the semiconductor switches for deciding the capacitance of the variable capacitor following the variation in the load impedance results in fast and continuous on-off operation of the semiconductor switches.

This operation raises a problem of increase in the amount of heat generated in the semiconductor switches due to switching loss.

It is an object of the present disclosure to provide an impedance matching device and an impedance matching method that are able to reduce the amount of heat generated in the semiconductor switches.

An impedance matching device according to one aspect of the present disclosure is an impedance matching device that is provided between a high-frequency power source and a load, and that obtains information concerning an impedance viewed from an output terminal of the high-frequency power source or from its equivalent to the output terminal toward a load side to thereby achieve impedance matching between the high-frequency power source and the load. The impedance matching device comprises: a variable capacitor including a plurality of capacitance elements respectively having capacitor each having one end connected to the high-frequency power source and semiconductor switch connected in series to the capacitor, the plurality of capacitance elements being connected in parallel; and a control unit that controls an on or off state of the semiconductor switches such that an impedance of the high-frequency power source is matched to an impedance of the load based on the information concerning the impedance obtained, and the control unit derives a reflection coefficient based on the information obtained concerning the impedance viewed from the high-frequency power source toward the load side, updates states of the respective semiconductor switches included in the plurality of capacitance elements with a first cycle in the case where the reflection coefficient is equal to or more than a predetermined value, and updates states of the respective semiconductor switches included in the plurality of capacitance elements with a second cycle longer than the first cycle in the case where the reflection coefficient is less than the predetermined value. Here, the part equivalent to the output terminal is the input terminal of the impedance matching device, for example.

According to the present aspect, the control unit updates the states of the respective semiconductor switches included in the plurality of capacitance elements with the first cycle in the case where the reflection coefficient is equal to or more than the predetermined value whereas the control unit updates the states of the respective semiconductor switches included in the plurality of capacitance elements with the second cycle longer than the first cycle in the case where the reflection coefficient is less than the predetermined value. Accordingly, after the reflection coefficient is less than the predetermined value, i.e., in a near-matched state, precise adjustment is required. However, updating with the second cycle that is longer than the first cycle can reduce the switching counts of the semiconductor switches, which reduces the heat generation.

In the impedance matching device according to one aspect of the present disclosure, the predetermined value for the reflection coefficient is 0.1.

According to the present aspect, the predetermined value for the reflection coefficient is set to 0.1, so that the value can be used as a threshold for determining whether or not a matched state is almost achieved.

In the impedance matching device according to one aspect of the present disclosure, the plurality of capacitance elements include a first capacitance element group and a second capacitance element group that includes the capacitors each having a capacitance smaller than a capacitance of one of the capacitors constituting the first capacitance element group, and the control unit updates states of the semiconductor switches included in the first capacitance element group and maintains states of the semiconductor switches included in the second capacitance element group at a time of updating with the first cycle, and updates states of the semiconductor switches included in the first capacitance element group and the semiconductor switches included in the second capacitance element group at a time of updating with the second cycle.

According to the present aspect, the plurality of capacitance elements include a first capacitance element group and a second capacitance element group that includes the capacitors each having a capacitance smaller than a capacitance of one of the capacitors constituting the first capacitance element group. The control unit updates the states of the semiconductor switches included in the first capacitance element group at a time of updating with the first cycle whereas the control unit updates the states of the semiconductor switches included in the first capacitance element group and the semiconductor switches included in the second capacitance element group at a time of updating with the second cycle. The first capacitance element group includes the capacitors each having a large capacitance, so that change of one capacitance causes great change in the impedance. Accordingly, during the unmatched state where the reflection coefficient is equal to or more than a predetermined value, rough adjustments are performed, so that an impedance is first adjusted by the first capacitance element group. If the reflection coefficient is less than the predetermined value (in the near-matched state), precise adjustments are required, so that the second capacitance element group is added to thereby adjust the impedance using the first capacitance element group and the second capacitance element group. The capacitors of the second capacitance element group having smaller capacitances than those of the capacitors constituting the first capacitance element group frequently repeat switching (updating the states of the semiconductor switches). The capacitor with particularly smaller capacitance performs switching highly frequently.

In contrast thereto, in the case of the unmatched state where the reflection coefficient is equal to or more than the predetermined value, the first capacitance element group is switched with the first cycle while in the case where the reflection coefficient is less than the predetermined value (in the near-matched state), the first capacitance element group and the second capacitance element group are switched with the second cycle, thereby updating the states of the semiconductor switches.

The second cycle is set to an cycle longer than the first cycle. Hence, if the reflection coefficient is less than the predetermined value (in the near-matched state), the update cycle is extended to thereby reduce the heat generation caused by the switching of the second capacitance element group. It is noted that after the near-matched state, the state of the first capacitance element group is scarcely changed, which lessens the influence of the heat generation due to the switching of the first capacitance element group.

That is, in the case where the reflection coefficient is less than the predetermined value (in the near-matched state), the update cycle is extended to thereby reduce the switching loss caused by the semiconductor switches being turned on or off, preventing the temperatures of the semiconductor switches from rising. In contrast, in the case of the unmatched state where the reflection coefficient is more than the predetermined value, the semiconductor switches of the first capacitance element group including the capacitors with large capacitances are updated with the first cycle, and thus the targeted impedance matching point can be reached soon.

In the impedance matching device according to one aspect of the present disclosure, a capacitance of any one of the capacitors included in the first capacitance element group is larger than a total value of capacitances of all the capacitors included in the second capacitance element group.

According to the present aspect, the capacitance of any one of the capacitors included in the first capacitance element group is larger than the total value of the capacitances of all the capacitors included in the second capacitance element group. Accordingly, if the capacitance required for the variable capacitor is larger than the total value of the capacitance of all the capacitors included in the second capacitance element group, this can be addressed by using the capacitors included in the first capacitance element group.

An impedance matching method according to one aspect of the present disclosure is for achieving impedance matching between a high-frequency power source and a load by a variable capacitor provided between the high-frequency power source and the load. The variable capacitor includes a plurality of capacitance elements having respective capacitors and respective semiconductor switches, and the method comprises: deriving a reflection coefficient based on information obtained concerning an impedance viewed from the high-frequency power source toward a load side; updating states of the respective semiconductor switches included in the plurality of capacitance elements with a first cycle in the case where the reflection coefficient is equal to or more than a predetermined value; and updating states of the respective semiconductor switches included in the plurality of capacitance elements with a second cycle longer than the first cycle in the case where the reflection coefficient is less than the predetermined value.

According to the present aspect, it is possible to provide the impedance matching method capable of reducing the amount of heat generated in the semiconductor switches.

It is possible to provide an impedance matching device and an impedance matching method that are able to reduce the amount of heat generated in the semiconductor switches.

The above and further objects and features will more fully be apparent from the following detailed description with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram roughly illustrating one example of the configuration of an impedance matching device according to Embodiment 1.

FIG. 2 is a block diagram roughly illustrating one example of the configuration of a control unit.

FIG. 3 is a circuit diagram roughly illustrating one example of the configuration of a driving circuit.

FIG. 4 is an illustrative view of a first capacitance element group.

FIG. 5 is an illustrative view of a second capacitance element group.

FIG. 6 is an illustrative view of an update cycle of semiconductor switches.

FIG. 7 is a flowchart illustrating processing performed by the control unit according to Embodiment 1.

FIG. 8 is a circuit diagram illustrating one example of the configuration of a driving circuit according to Modification 1.

MODE FOR CARRYING OUT DISCLOSURE Embodiment 1

The present disclosure will be described below with reference to the drawings depicting embodiments thereof. FIG. 1 is a circuit diagram roughly illustrating one example of the configuration of an impedance matching device 1 according to Embodiment 1. The impedance matching device 1 is connected between a high-frequency power source 6 and a load 8. Between the high-frequency power source 6 and the impedance matching device 1, a high-frequency detection unit 7 is provided. That is, the high-frequency detection unit 7 is provided between the output terminal of the high-frequency power source 6 and the input terminal of the impedance matching device 1. The impedance matching device 1 obtains information concerning an impedance (load-side impedance) viewed from the high-frequency power source 6 toward the load 8 side that is detected by the high-frequency detection unit 7, and matches the impedance viewed from the high-frequency power source 6 toward the load 8 side to the output impedance of the high-frequency power source 6 (makes the load-side impedance and the output impedance of the high-frequency power source 6 conjugate with each other) based on the information.

The high-frequency power source 6 is an alternating current (AC) power source that outputs high-frequency power at the radio frequency (RF) band, for example, of 2 MHz, 13.56 MHz, 27 MHz or 60 MHz or the like, and has an output impedance set to a predetermined value such as 50Ω, for example. The high-frequency power source 6 includes an inverter circuit (not illustrated), and performs switching control of the inverter circuit at high speeds to thereby generate AC power at high frequencies and outputs the generated AC power to the load 8.

The load 8 is for performing various processing using the high-frequency power input from the high-frequency power source 6 and includes, for example, a plasma processing apparatus, contactless power transfer device, or the like. In the plasma processing apparatus, the state of the plasma varies with time as the manufacturing process such as plasma etching, plasma CVD, etc. advances. This varies the impedance of the load 8. In order to efficiently supply power to the load 8 from the high-frequency power source 6, the impedance (load-side impedance) viewed from the high-frequency power source 6 toward the load 8 side needs to be adjusted as the impedance of the load 8 varies.

The high-frequency detection unit 7 detects parameters used for calculating the load-side impedance, which is an impedance viewed from the output terminal of the high-frequency power source 6 toward the load 8 side. The parameters are derived using high-frequency voltage and high-frequency current that are supplied from the high-frequency power source 6 to the load 8 as well as a phase difference between the high-frequency voltage and the high-frequency current, for example. The parameters include forward wave voltage, reflected wave voltage, forward wave current, reflected wave current, forward wave power and reflected wave power that are detected at the connection node between the high-frequency detection unit 7 and the impedance matching device 1 or at the output terminal of the high-frequency power source 6, or include a reflection coefficient derived by using the forward wave voltage, etc. and the reflected wave voltage, etc. The matter concerning the reflection coefficient is based on a known art (Japanese Patent Application Laid-Open No. 2014-236435), for example. The parameters include transmission parameters (T parameters) or scattering parameters (S parameters) as disclosed in a known document (Japanese Patent Application Laid-Open No. 2006-166412). The high-frequency detection unit 7 corresponds to an impedance detection unit that detects information concerning the impedance (load-side impedance) viewed from the high-frequency power source 6 toward the load 8 side. Note that the load-side impedance corresponds to a complex impedance of the impedance of the load 8 and the impedance of the impedance matching device 1, and thus the impedance of the load 8 may be derived by subtracting the impedance of the impedance matching device 1 from the load-side impedance.

The impedance matching device 1 includes two variable capacitors 2, an inductor 5, driving circuits 3 and a control unit 4. The impedance matching device 1 changes the capacitance of the variable capacitors 2 based on the information concerning the load-side impedance detected by the high-frequency detection unit 7 and adjusts the load-side impedance, to thereby achieve impedance matching. Moreover, the impedance matching device 1 derives a reflection coefficient based on the information concerning the load-side impedance detected by the high-frequency detection unit 7 and controls update cycles of the variable capacitors 2 based on the derived reflection coefficient.

In the impedance matching device 1, one of the variable capacitors 2 has one end connected to the high-frequency power source 6 via the high-frequency detection unit 7 and the other end connected to the ground (grounded). The other one of the variable capacitor 2 is connected to the one variable capacitor 2 in parallel and has one end connected to the high-frequency power source 6 via the high-frequency detection unit 7 and the other end connected to the load 8. Between the other variable capacitor 2 and the load 8, the inductor 5 is connected in series. That is, the connection between the variable capacitor 2 and the high-frequency power source 6 is not limited to the direct connection and includes indirect connection via the high-frequency detection unit 7 or the like as well. These two variable capacitors 2 and the inductor 5 constitute a reverse L-shaped matching circuit 11. Note that the matching circuit 11 may be, though not limited to the reverse L-shaped matching circuit 11, a π typed matching circuit 11 in which two variable capacitors 2 each having one end connected to the high-frequency power source 6 and the other end connected to the ground (grounded) are provided so as to be connected in parallel.

The variable capacitors 2 each include a first capacitance element group 21 and a second capacitance element group 22. The first capacitance element group 21 includes multiple first capacitance elements 211 that are connected in parallel to each other. The first capacitance elements 211 each include a higher-order bit capacitor 212 that has one end connected to the high-frequency power source 6 side and a higher-order bit switch 213 that is connected in series to the other end of the higher-order bit capacitor 212. The capacitances of the respective higher-order bit capacitors 212 are set to the same.

The higher-order bit switch 213 is, for example, a PIN diode having the anode connected to the high-frequency power source 6 side and the cathode connected to the ground side. To the connection node between the anode of the PIN diode and the other end of the higher-order bit capacitor 212, an electric wire to which DC voltage from the driving circuit 3 as described below is output is connected. The DC voltage output from the driving circuit 3 is applied in a forward direction between the anode and the cathode of the PIN diode, to thereby turn the PIN diode to the on state. The higher-order bit switch 213 formed by the PIN diode is turned to the on state, to thereby allow high-frequency current to bidirectionally flow through the higher-order bit switch 213. Furthermore, the higher-order bit switch 213 is turned to the on state to thereby allow the higher-order bit capacitor 212 connected in series to the higher-order bit switch 213 to work. Depending on the number of higher-order bit switches 213 that are turned to the on state, the capacitance of the first capacitance element group 21 (the capacitance of the single higher-order bit capacitor 212*the number of higher-order bit switches 213 that are turned to the on state) can be adjusted.

The second capacitance element group 22 includes multiple second capacitance elements 221 that are connected in parallel to each other. The second capacitance elements 221 each include a lower-order bit capacitor 222 having one end connected to the high-frequency power source 6 side and a lower-order bit switch 223 connected in series to the other end of the lower-order bit capacitor 222.

The capacitances of the respective lower-order bit capacitors 212 are set to be different from each other in a stepwise manner. Out of the lower-order bit capacitors 222, the lower-order bit capacitors 222 other than the lower-order bit capacitor 222 with the smallest capacitance (the smallest capacitor) are set to values obtained by multiplying the capacitance of the smallest capacitor by powers of two. The details of the relationship between the capacitances of the respective lower-order bit capacitors 222 and the capacitances of the higher-order bit capacitors 212 or the like will be described later.

Similarly to the higher-order bit switch 213, the lower-order bit switch 223 is a PIN diode having the anode connected to the high-frequency power source 6 side and the cathode connected to the ground side, and is turned to the on state in response to the DC voltage output from the driving circuit 3. The lower-order bit switch 223 is turned to the on state to thereby allow the lower-order bit capacitor 222 connected in series to the lower-order bit switch 223 to work. Depending on the states of the lower-order bit switches 223 that are turned to the on state, the capacitance of the second capacitance element group 22 (the total value of the capacitances of the lower-order bit capacitors 222 that are turned to the on state) can be adjusted.

It is noted that in the present embodiment, three first capacitance elements 211 included in the first capacitance element group 21 are provided while four second capacitance elements 221 included in the second capacitance element group 22 are provided. The number of capacitance elements may however take any number, not limited to the above. In some embodiments, two or more first capacitance elements 211 may be provided while two or more second capacitance elements 221 may be provided. In FIG. 1, the variable capacitor 2 is constituted by, though not limited to, the first capacitance element group 21 and the second capacitance element group 22. The variable capacitor 2 may include a capacitance element other than the first capacitance element group 21 and the second capacitance element group 22. In FIG. 1, the impedance matching device 1 is configured to include two variable capacitors 2 in order to adjust the capacitance though the number of variable capacitors 2 may however take any number, not limited to the above. The impedance matching device 1 may include one, or three or more variable capacitors 2 and further include a capacitor with a fixed capacitance, in order to adjust the capacitance.

FIG. 2 is a block diagram roughly illustrating one example of the configuration of the control unit 4. The control unit 4 is constituted by a central processing unit (CPU), a micro processing unit (MPU) or the like, and has a clock function and a calculation function. The control unit 4 is connected to a storage unit 40 such as a read only memory (ROM), a random access memory (RAM) or the like through an internal bus or the like, and executes a program stored in the storage unit 40 to thereby function as an impedance calculation unit 41, a capacitance calculation unit 42, a targeted switch-state decision unit 43 and an update cycle decision unit 44. The control unit 4, which includes a switch control unit 45, obtains information concerning the impedance (load-side impedance) viewed from the high-frequency power source 6 toward the load 8 side that is detected by the high-frequency detection unit 7 and controls the switch control unit 45 based on the information to thereby adjust the capacitance of the variable capacitor 2, achieving the impedance matching.

The impedance calculation unit 41 is a function unit for obtaining information concerning the impedance viewed from the high-frequency power source 6 toward the load 8 side that is detected and output by the high-frequency detection unit 7 or the parameters including the information at a predetermined cycle to thereby calculate the impedance viewed from the high-frequency power source 6 toward the load 8 side as load-side impedance. The cycle (calculation cycle) for calculating the load-side impedance is, for example, 100 μsec to 1 msec. Moreover, the impedance calculation unit 41 obtains information concerning the impedance viewed from the high-frequency power source 6 toward the load 8 side that is detected and output by the high-frequency detection unit 7 or the parameters including the information at predetermined cycles, and derives a reflection coefficient. As described above, the parameters detected by the high-frequency detection unit 7 include the reflection coefficient, and thus the impedance calculation unit 41 may derive the reflection coefficient from the parameters obtained from the high-frequency detection unit 7. Alternatively, the impedance calculation unit 41 may calculate the reflection coefficient (reflection coefficient=reflected wave voltage/forward wave voltage) based on the forward wave voltage, the reflected wave voltage, etc. included in the parameters. That is, the impedance calculation unit 41 corresponds to a reflection coefficient derivation unit.

The capacitance calculation unit 42 is a function unit for calculating the capacitance required for the variable capacitor 2 in order to match the load-side impedance calculated by the impedance calculation unit 41 to the output impedance of the high-frequency power source 6.

The targeted switch-state decision unit 43 is a function unit for deciding the on or off state of the respective PIN diodes (the higher-order bit switch 213 or the lower-order bit switch 223, or the higher-order bit switch 213 and the lower-order bit switch 223) included in the variable capacitor 2 in order to match the capacitance of the variable capacitor 2 to the capacitance calculated by the capacitance calculation unit 42.

The update cycle decision unit 44 is a function unit for obtaining a reflection coefficient derived by the impedance calculation unit 41 and deciding the cycle (update cycle) to update the state of the higher-order bit switch 213 or the lower-order bit switch 223 based on the reflection coefficient. The update cycle is, for example, 100 μsec to 5 msec, and the update cycle may be equalized with the load-side impedance calculation cycle or the reflection coefficient derivation cycle (calculation cycle). The details of the update cycle decision unit 44 will be described later using the flowchart, which will be described later.

The switch control unit 45 includes a control circuit 451 and a branch circuit 452 (see FIG. 3). The switch control unit 45 updates the state of the higher-order bit switch 213 or the lower order bit switch 223, or the states of the higher-order bit switch 213 and the lower-order bit switch 223 at the update cycle decided by the update cycle decision unit 44. The switch control unit 45 outputs to the driving circuit 3 an on-off signal for updating the state of the higher-order bit switches 213 or the lower-order bit switches 223 based on the state of the higher-order bit switch 213 or the lower-order bit switch 223 decided by the targeted switch-state decision unit 43, and updates the state of the higher-order bit switch 213 or the lower-order bit switch 223.

FIG. 3 is a circuit diagram roughly illustrating one example of the configuration of the driving circuit 3. The driving circuit 3 is provided for each of the higher-order bit switches 213 or each of the lower-order bit switches 223 as illustrated in FIG. 1. It is noted that FIG. 3 mainly illustrates the first capacitance element 211.

The one end of the driving circuit 3 is connected to the connection node between the higher-order bit capacitor 212 or the lower-order bit capacitor 222 and the anode of the PIN diode (the higher-order bit switch 213 or the lower-order bit switch 223). The other end of the driving circuit 3 is connected to the switch control unit 45 included in the control unit 4. The driving circuit 3 outputs DC voltage from the one end based on a control signal output from the switch control unit 45 and applies the DC voltage in a forward direction of the PIN diode (the higher-order bit switch 213 or the lower-order bit switch 223).

The driving circuit 3 includes a high-side switch 31 and a low-side switch 32. The high-side switch 31 and the low-side switch 32 are semiconductor switches, for example, n-type metal-oxide-semiconductor field-effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs) or the like.

The source of the high-side switch 31 and the drain of the low-side switch 32 are connected in series, and the connection node between the high-side switch 31 and the low-side switch 32 as well as the connection node between the PIN diode and the higher-order bit capacitor 212 or the lower-order bit capacitor 222 are connected by an electric wire (output line). The output line is provided with a filter circuit 33 (low-pass filter) constituted by a capacitor FC and a coil FL. Between the high-side switch 31 and the connection node of the high-side switch 31 and the low-side switch 32, a resistor R and a speed-up capacitor SC are connected in parallel. The drain of the high-side switch 31 is connected to the positive output terminal 34 of the driving source (V). The source of the low-side switch 32 is connected to the negative output terminal 35 of the driving source (V). The respective gate terminals of the high-side switch 31 and the low-side switch 32 are connected to two signal lines extending from the switch control unit 45.

The switch control unit 45 includes the control circuit 451 and the branch circuit 452, and outputs a control signal for complementary turning on or off the high-side switch 31 and the low-side switch 32 to signal lines divided into two branches at the branch circuit 452. In other words, based on the control signal output from the control circuit 451, the branch circuit 452 generates two voltage signals that are opposite to each other between the high level (H) and the low-level (L) to output the one voltage signal to the high-side switch 31 and the other voltage signal to the low-side switch 32. The high-side switch 31 to which a high level (H) voltage signal is input is turned to the on state while the low-side switch 32 to which a low level (L) voltage signal is input is turned to the off state. Alternatively, the high-side switch 31 to which a low level (L) voltage signal is input is turned to the off state while the low-side switch 32 to which a high level (H) voltage signal is input is turned to the on state. Hence, the high-side switch 31 and the low-side switch 32 are complementary controlled so as to be in the on or off state, so that one of the switches is in the on state while the other is in the off state.

When the high-side switch 31 is in the on state (the low-side switch 32 is in the off state), the positive side (V+) of the driving source and the anode of the PIN diode are connected to each other, so that DC voltage in the forward direction (voltage necessary for causing current in the forward direction to flow) is applied to the anode of the PIN diode, turning the PIN diode to the on state. When the high-side switch 31 is in the off state (the low-side switch 32 is in the on state), the negative side (V−) of the driving source and the anode of the PIN diode are connected to each other, so that negative voltage (voltage in a rage where breakdown voltage does not occur) is applied to the anode of the PIN diode, turning the PIN diode to the off state.

FIG. 4 is an illustrative view of the first capacitance element group 21. FIG. 5 is an illustrative view of the second capacitance element group 22. In the present embodiment, the first capacitance element group 21 of the variable capacitor 2 includes three first capacitance elements 211 while the second capacitance element group 22 includes four second capacitance elements 221. It is understood that the number of first capacitance elements 211 and the number of second capacitance elements 221 are not limited to the above-mentioned exemplified numbers.

The capacitances of the higher-order bit capacitors 212 (HCs 1-3) of the respective three first capacitance elements 211 are all the same value of 16 pF. The higher-order bit capacitors 212 are connected in parallel, so that the total value of the capacitances of the higher-order bit capacitors 212 for which corresponding higher-order bit switches 213 are turned on is the capacitance of the first capacitance element group 21. It is noted that the same capacitance value does not necessarily mean that the capacitances are not required to be strictly the same, and a margin of error within the rated value of the capacitor, for example, may be allowed. That is, when the capacitance of the first capacitance element group 21 is adjusted, the capacitances of the higher-order bit capacitors 212 may take the same value within the allowable range in terms of precision.

The capacitances of the lower-order bit capacitors 222 (LCs 1-4) of the respective four second capacitance elements 221 are different from each other in a stepwise manner. Out of these capacitors 222, the lower-order bit capacitor 222 (hereinafter referred to as the smallest capacitor) with the smallest capacitance has a value of 1 pF, for example. Meanwhile, the capacitances of the other three lower-order bit capacitors 222 are set to increase in a stepwise manner (in binary steps) so as to take values obtained by multiplying the capacitance of the smallest capacitor by the powers of two. As illustrated in FIG. 5, the lower-order bit capacitors 222 are assigned with the number of digits 1 to 4 from the smallest capacitor 222, and the digit numbers are regarded as bit numbers. By combining the on or off state of the lower-order bit capacitors 222 for which capacitances are set in binary steps, the total value (0 to 15 pF) of the capacitances can be adjusted at 16 levels (two to the fourth power). That is, assuming that the capacitance of the smallest capacitor is Cmin [pF], and the number of lower-order bit capacitors 222 is n, the capacitance Ck of the lower-order bit capacitor 222 whose digits in number (bit number) is k (here, 1≤K≤n) is represented by a value (Ck=Cmin×2(k−1)) obtained by multiplying Cmin by 2 to the power of (k−1). The lower-order bit capacitors 222 are connected in parallel to each other, so that the total value of the capacitances of the lower-order bit capacitors 222 for which corresponding lower-order bit switches 223 are turned on is the capacitance of the second capacitance element group 22. The capacitances of the lower-order bit capacitors 222 are set in binary steps, and thus the respective bits are represented by 1 or 0, so that the capacitance of the second capacitance element group 22 can be adjusted at each step of 2 to the power of n (n: the number of lower-order bit capacitors 222).

The capacitance of the variable capacitor 2 is the total value of the capacitance of the first capacitance element group 21 and the capacitance of the second capacitance element group 22. The capacitance of a single higher-order bit capacitor 212 is set to a value larger than the total value of the capacitances of all the lower-order bit capacitors 222. Moreover, the capacitance of each of the higher-order bit capacitors 212, that is, the capacitance of a single higher-order bit capacitor 212 is set to a value obtained by multiplying the capacitance of the smallest capacitor by 2 to the power of n (n: the number of lower-order bit capacitors 222). The capacitance of the higher-order bit capacitors 212 and the lower-order bit capacitors 222 are thus set, and even if the capacitance required for the variable capacitor 2 exceeds the total value of the capacitances of all the lower-order bit capacitors 222, this can be addressed by turning the higher-order bit switch 213 to the on state. In the present embodiment, if the required capacitance is 18 pF, for example, this can be addressed by combining one higher-order bit capacitor 212 (16 pF) and the lower-order bit capacitor 222 (2 pF) corresponding to 2 bits. The capacitance of each of the higher-order bit capacitors 212, that is, the capacitance of a single higher-order bit capacitor 212 is set to a value obtained by multiplying the capacitance of the smallest capacitor by 2 to the power of n (n: the number of lower-order bit capacitors 222). Hence, by combining the higher-order bit capacitors 212 and the lower-order bit capacitors 222, the capacitance of the variable capacitor 2 can be increased in a stepwise manner. It is noted that the capacitance of each of the higher-order bit capacitors 212 is required to be set to a large value to some extent. An excessive large value, however, may complicate the control, and thus a moderate value is more preferable.

FIG. 6 is an illustrative view of an update cycle of the semiconductor switch. The horizontal axis indicates time while the vertical axis indicates a signal waveform corresponding to the update cycle. The update cycle includes a first cycle (t1) and a second cycle (t2). The first cycle (t1) is set to be shorter than the second cycle (t2) (t1<t2).

The control unit 4 updates each of the higher-order bit switches 213 with the first cycle (t1) directly after starting impedance matching operation. When updating the higher-order bit switch 213, the control unit 4 outputs a control signal indicating the on or off state of the higher-order bit switch 213 for every first cycle (t1). As illustrated in FIG. 4, in the case where three higher-order bit switches 213 (HC1, HC2, HC3) are provided, the control signal includes information indicating the on or off states corresponding to the three higher-order bit switches 213. For example, in the case where the control signal represents “101,” the higher-order bit switch 213 (HC3) is turned on, the higher-order bit switch 213 (HC2) is turned off, and the higher-order bit switch 213 (HC1) is turned on. Alternatively, in the case where the control signal represents “000,” all the higher-order bit switches 213 are turned off, while in the case where the control signal represents “111,” all the higher-order bit switches 213 are turned on. During updating with the first cycle (t1), the control unit 4 does not output a control signal indicating the on or off state of the lower-order bit switches 223, or continuously outputs a control signal indicating that all the lower-order bit switches 223 are turned on. Accordingly, during updating with the first cycle (t1), the states of the lower-order bit switches 223 are fixed. During updating with the first cycle (t1), the states of the lower-order bit switches 223 are fixed, which enables reduction in the heat generation due to the switching loss of the lower-order bit switches 223.

If a control signal previously output and a control signal currently output are different from each other, the state of any of the higher-order bit switch 213 is changed from the previous time to this time. If a control signal previously output and a control signal currently output are the same, the states of the higher-order bit switches 213 remain unchanged from the previous time to this time.

In the case where the load-side impedance is close to a targeted matching point (for example, the reflection coefficient is less than 0.1), the control unit 4 changes the update cycle from the first cycle (t1) to the second cycle (t2). That is, by setting 0.1, for example, as a predetermined value (threshold) for changing the update cycle from the first cycle (t1) to the second cycle (t2) based on the reflection coefficient, the control unit 4 changes the update cycle to the second cycle (t2) that is longer than the first cycle (t1) after the load-side impedance is close to the targeted matching point.

During updating with the second cycle (t2), the control unit 4 outputs control signals indicating the on or off state of the higher-order bit switches 213 and the lower-order bit switches 223 at every second cycle (t2). Even if update is performed multiple times with the second cycle (t2), the states of the higher-order bit switches 213 are generally maintained, enabling reduction in the heat generation due to the switching loss of the higher-order bit switches 213. The second cycle (t2) is longer than the first cycle (t1), and thus the switching count of each of the lower-order bit switches 223 is reduced, enabling reduction in the heat generation due to the switching loss of the lower-order bit switches 223. After the targeted matching point is achieved (for example, the reflection coefficient is less than 0.03), the control unit 4 continues to update the states of the higher-order bit switches 213 and the lower-order bit switches 223 with the second cycle (t2).

In the case where the reflection coefficient is equal to or more than a predetermined value (threshold), for example, directly after impedance matching operation is started, impedance matching is performed at the shorter first cycle (t1) by the higher-order bit capacitors 212 having large capacitances, whereby the targeted matching point can be achieved in a short time period while the heat generation of the lower-order bit switches 223 is reduced. In the case where the reflection coefficient is less than the predetermined value (threshold), the load-side impedance is in a near-matched state, and thus the states of the higher-order bit switches 213 are scarcely changed and are maintained, which lessens the influence of the heat generation due to the switching of the higher-order bit switches 213. It is noted that in the case where the reflection coefficient is equal to or more than the predetermined value (threshold), the higher-order bit switches 213 are updated with the first cycle (t1) while the states of the respective lower-order bit switches 223 are fixed, though the states of the switches are not limited thereto. Even in the case where the reflection coefficient is equal to or more than the predetermined value (threshold), the higher-order bit switches 213 and the lower-order bit switches 223 may be updated with the first cycle (t1). In the case where the reflection coefficient is equal to or more than the predetermined value (threshold), i.e., in an impedance mismatched state, rough adjustments are preferentially performed, and thus the higher-order bit switches 213 are mainly changed to allow the load-side impedance to be close to the matching point in a relatively short time period, making the reflection coefficient less than the predetermined value (threshold). Hence, even if the states of the lower-order bit switches 223 are updated with the first cycle (t1), the influence of the heat generation can be lessened.

FIG. 7 is a flowchart illustrating processing performed by the control unit 4 according to Embodiment 1. The control unit 4 of the impedance matching device 1 starts impedance matching operation in response to a matching operation starting instruction, or the like.

The control unit 4 turns the lower-order bit switch 223 off (S11). The control unit 4 turns all the lower-order bit switches 223 of the second capacitance element group 22 included in the variable capacitor 2 to the off state. All the lower-order bit switches 223 are turned to the off state to thereby disable all the lower-order bit capacitors 222 of the second capacitance element group 22, resulting in 0 pF of the capacitance of the second capacitance element group. By turning all the lower-order bit switches 223 to the off state, the heat generation of the lower-order bit switches 223 can be reduced. It is noted that all the lower-order bit switches 223 may be turned to another state, not turned to the off state. For example, the states of the lower-order bit switches 223 at a time when the previous impedance matching operation was completed are stored, and the stored states may be employed.

The control unit 4 sets the update cycle to the first cycle (S12). The control unit 4 refers to the first cycle stored in advance in the storage unit 40 and sets the first cycle as an update cycle. The processing at step S16, which will be described later, is performed at this first update cycle.

The control unit 4 obtains information concerning an impedance, and calculates (derives) an impedance and a reflection coefficient (S13).

The control unit 4 obtains information (parameters) concerning the impedance viewed from the high-frequency power source 6 toward the load 8 side that is detected and output by the high-frequency detection unit 7, and calculates an impedance viewed from the high-frequency power source 6 toward the load 8 side as a load-side impedance. Furthermore, the control unit 4 calculates (derives) a reflection coefficient based on the parameters.

The control unit 4 calculates a capacitance (S14). The control unit 4 calculates the capacitance required for the variable capacitor 2 in order to match the calculated load-side impedance to the output impedance of the high-frequency power source 6.

The control unit 4 decides a targeted state of the switches (S15). The control unit 4 decides the on or off state of the higher-order bit switches 213 and the lower-order bit switches 223 included in the variable capacitor 2 as a targeted state of the switches in order to match the capacitance of the variable capacitor 2 to the capacitance calculated by the capacitance calculation unit 42. Alternatively, the control unit 4 may decide the on or off state of only the higher-order bit switches 213.

The control unit 4 updates the states of the higher-order bit switches 213 (S16). The control unit 4 extracts the states of the higher-order bit switches 213 in the targeted state of the switches decided and updates the on or off state of the higher-order bit switches 213 with the first cycle. The control unit 4 stores the updated on or off state of the higher-order bit switches 213 in the storage unit 40 in association with the time point when updating is performed.

The control unit 4 determines whether or not the reflection coefficient is less than a predetermined value (S17). The control unit 4 compares the reflection coefficient (the current reflection coefficient) calculated at step S13 and the predetermined value for the reflection coefficient stored in the storage unit 40 in advance, and determines whether or not the current reflection coefficient is less than the predetermined value. The predetermined value for the reflection coefficient is, for example, 0.1. By setting the reflection coefficient to 0.1, this can be used as a threshold for determining whether or not a matched-state is almost achieved.

If the reflection coefficient is not less than the predetermined value (S17: No), that is, if the reflection coefficient is equal to or more than the predetermined value, the control unit 4 performs loop processing in order to execute the processing from step S13 again. Accordingly, updating with the first cycle is continued.

If the reflection coefficient is less than the predetermined value (S17: Yes), the control unit 4 sets the update cycle to the second cycle (S18). The control unit 4 refers to the second cycle stored in advance in the storage unit 40 and sets the second cycle as an update cycle. The processing at step S22, which will be described later, is performed with the second cycle. The second cycle is an cycle longer than the first cycle. By setting the second cycle to an cycle two-times longer than the first cycle, for example, the switching count upon updating can be reduced, which enables efficient reduction in heat generation.

The control unit 4 performs processing from S19 to S21 similarly to the processing from S13 to S15. It is noted that in the processing at S21, the control unit 4 decides the states of the higher-order bit switches 213 and the lower-order bit switches 223 as a targeted state of the switches.

The control unit 4 updates the states of the higher-order bit switches 213 and the lower-order bit switches 223 (S22). The control unit 4 updates the on or off state of the higher-order bit switches 213 and the lower-order bit switches 223 with the second cycle based on the decided targeted state of the switches. The control unit 4 stores the updated on or off state of the higher-order bit switches 213 and the lower-order bit switches 223 in the storage unit 40 in association with the time point of updating.

The control unit 4 determines whether or not the reflection coefficient is less than the predetermined value (S23). Similarly to the processing at step S17, the control unit 4 compares the reflection coefficient (the current reflection coefficient) calculated at step S19 and the predetermined value for the reflection coefficient stored in the storage unit 40 in advance, and determines whether or not the current reflection coefficient is less than the predetermined value.

As described above, the total value (15 pF, for example) of the capacitances of all the lower-order bit capacitors 222 is smaller than the capacitance (16 pF, for example) of a single higher-order bit capacitor 212. Accordingly, if the required capacitance of the variable capacitor 2 varies from, for example, 31 pF to 32 pF upon one updating, the number of higher-order bit switches 213 that are to be turned on is increased from one to two, and the lower-order bit switches 223 that are all in the on state are all turned to the off state. Such a phenomenon in which one higher-order bit switch 213 changes its state is called carrying (or borrowing). That is, the state of one higher-order bit switch 213 is changed for one updating, however, the amount of change in the required capacitance of the variable capacitor 2 is a small value, and the load-side impedance is close to the matching point. In contrast thereto, in the case where the states of two or more higher-order bit switches 213 are changed, the amount of change in the required capacitance of the variable capacitor 2 is a large value, and the load-side impedance is away from the matching point. When it is determined whether or not the load-side impedance achieves a matched state once and changes to an unmatched state again (a state where the load-side impedance is away from the matching point) by carrying or borrowing, the capacitances of the multiple higher-order bit capacitors 212 in the first capacitance element group 21 are set to the same values as described above.

If the reflection coefficient is not less than the predetermined value (S23: NO), the control unit 4 performs loop processing in order to execute the processing at step S12 again. Accordingly, the update cycle is changed to the first cycle to update the higher-order bit switches 213. If the reflection coefficient is not less than the predetermined value, that is, if the reflection coefficient is equal to or more than the predetermined value, the load-side impedance is away from the matching point (in a mismatched state), rough adjustments are preferentially performed, and the impedance is adjusted by the higher-order bit switches 213 (first capacitance element group 21). In other words, the control unit 4 sets (returns) the update cycle to the first cycle with a shorter update cycle to fix the states of the lower-order bit switches 223 as well as to update the states of the higher-order bit switches 213, which allows the load-side impedance to be close to the matching point soon by the higher-order bit capacitors 212 with large capacitance. During updating with the first cycle, the states of the lower-order bit switches 223 are fixed (maintained), which eliminates the switching loss of the lower-order bit switches 223, enabling reduction in the heat generation.

If the reflection coefficient is less than the predetermined value (S23: Yes), the control unit 4 performs loop processing in order to execute the processing at step S19 again. Thus, updating with the second cycle is continued.

In the present embodiment, the update cycle is changed depending on whether or not the reflection coefficient is less than the predetermined value, though the basis for change is not limited thereto. The control unit 4 may change the update cycle depending on whether or not reflected power, reflected voltage or reflected current is less than a predetermined value. The smaller the reflection coefficient is, the smaller the reflected power, reflected voltage or reflected current is. Thus, the control unit 4 may perform an update with the second cycle if the reflected power, reflected voltage or reflected current is less than the predetermined value while performing an update with the first cycle if the reflected power, reflected voltage or reflected current is equal to or more than the predetermined value.

Modification 1

FIG. 8 is a circuit diagram roughly illustrating one example of the configuration of a driving circuit 3 according to Modification 1. It is noted that FIG. 8 mainly illustrates the first capacitance element 211. In Embodiment 1, the control circuit 451 outputs respective two voltage signals that are opposite to each other to the high-side switch 31 and the low-side switch 32, though the signals to be output are not limited thereto. The control circuit 451 may be configured to output a voltage signal (common input signal) common to the high-side switch 31 and the low-side switch 32.

The driving circuit 3 in Modification 1 is provided with a high-side switch 31 and a low-side switch 32 similarly to the driving circuit 3 in Embodiment 1. The high-side switch 31 is constituted by a photocoupler. The low-side switch 32 is constituted by an n-type MOSEFT similarly to Embodiment 1.

Similarly to Embodiment 1, the emitter of the high-side switch (photocoupler) 31 is connected in series to the drain of the low-side switch 32, and the connection node between the high-side switch (photocoupler) 31 and the low-side switch 32 is connected to the connection node between a higher-order bit capacitor 212 or a lower-order bit capacitor 222 and a PIN diode (a higher-order bit switch 213 or a lower-order bit switch 223) by a connection line (output line). The output line is provided with a filter circuit 33 (low-pass filter) constituted by a capacitor FC and a coil FL.

Similarly to Embodiment 1, the collector of the high-side switch (photocoupler) 31 is connected to the positive output terminal 34 of a driving source (V) while the source of the low-side switch (FET) 32 is connected to the negative output terminal 35 of the driving source (V).

The anode of the photodiode of the high-side switch (photocoupler) 31 is connected to a 5-V DC power source, for example, while the cathode thereof is connected to the gate of the low-side switch (FET) 32.

The control circuit 451 included in a switch control unit 45 and the connection node between the cathode of the photodiode and the gate of the low-side switch (FET) 32 are connected by a signal line. The switch control unit 45 outputs a voltage signal (5 V or larger, for example) larger than the output voltage of the DC power source connected to the photodiode as a voltage signal at the high level (H) to thereby turn the high-side switch (photocoupler) 31 to the off state and turn the low-side switch (FET) 32 to the on state. The switch control unit 45 outputs a voltage signal of 0 V as a voltage signal at the low level (L) to thereby turn the high-side switch (photocoupler) 31 to the on state and the low-side switch (FET) 32 to the off state.

The high-side switch (photocoupler) 31 is turned to the on state to thereby turn the PIN diode to the on state. The low-side switch (FET) 32 is turned to the on state to thereby turn the PIN diode to the off state.

In Modification 1 and Embodiment 1 illustrated in FIG. 3, the anode of the PIN diode (the higher-order bit switch 213 or the lower-order bit switch 223) is connected to the higher-order bit capacitor 212 or the lower-order bit capacitor 222 while the cathode thereof is grounded (connected to the ground), though the configuration of the PIN diode is not limited thereto. The anode of the PIN diode (the higher-order bit switch 213 or the lower-order bit switch 223) may be grounded while the cathode thereof may be connected to the higher-order bit capacitor 212 or the lower-order bit capacitor 222. Here, in the case where the positive side (V+) of the driving source is connected to the connection node (cathode of the PIN diode) between the PIN diode and the higher-order bit capacitor 212 or the lower-order bit capacitor 222 (the high-side switch 31 is in the on state and the low-side switch 32 is in the off state), the PIN diode is turned off. In the case where the negative side (V−) of the driving source is connected to the cathode of the PIN diode (the high-side switch 31 is in the off state and the low-side switch 32 is in the on state), the PIN diode is turned on.

Hence, a common input signal being the same voltage signal is output from the control circuit 451 to the high-side switch (photocoupler) 31 and the low-side switch 32 through one signal line, whereby the high-side switch (photocoupler) 31 and the low-side switch 32 can complementary be controlled between on and off. A common input signal is output from the control circuit 451 through one signal line, which eliminates the need for the branch circuit 452 in Embodiment 1.

It is to be understood that the embodiments disclosed here is illustrative in all respects and not restrictive. The scope of the present invention is defined by the appended claims, and all changes that fall within the meanings and the bounds of the claims, or equivalence of such meanings and bounds are intended to be embraced by the claims.

It is to be noted that, as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.

It is to be noted that the disclosed embodiment is illustrative and not restrictive in all aspects. The scope of the present invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims 

What is claimed is:
 1. An impedance matching device to achieve impedance matching between a high-frequency power source and a load, the impedance matching device comprising: a variable capacitor including a plurality of capacitance elements respectively having capacitor each having one end connected to the high-frequency power source and semiconductor switch connected in series to the capacitor, the plurality of capacitance elements being connected in parallel; and a control unit coupled to an impedance information detection unit located between the high-frequency power source and the impedance matching device to generate tuning signals to control an on or off state of the each of semiconductor switches such that an output impedance of the high-frequency power source is matched to an impedance of the load based on the matching information concerning the impedance obtained, wherein the control unit generates the tuning signals by calculating a reflection coefficient based on the matching information obtained by the impedance information detection unit concerning the impedance viewed from the high-frequency power source toward a load side of the load, updates the on or off states of the semiconductor switches included in the plurality of capacitance elements with a first update cycle where the reflection coefficient is equal to or more than a predetermined value, and updates the on or off states of the semiconductor switches included in the plurality of capacitance elements with a second update cycle longer than the first cycle where the reflection coefficient is less than the predetermined value.
 2. The impedance matching device according to claim 1, wherein the predetermined value for the reflection coefficient is 0.1.
 3. The impedance matching device according to claim 1, wherein the plurality of capacitance elements include a first capacitance element group and a second capacitance element group that includes the capacitors each having a capacitance smaller than a capacitance of one of the capacitors constituting the first capacitance element group, wherein the control unit updates states of the semiconductor switches included in the first capacitance element group and maintains states of the semiconductor switches included in the second capacitance element group at a time of updating with the first update cycle, and updates states of the semiconductor switches included in the first capacitance element group and the semiconductor switches included in the second capacitance element group at a time of updating with the second update cycle.
 4. The impedance matching device according to claim 3, wherein a capacitance of any one of the capacitors included in the first capacitance element group is larger than a total value of capacitances of all the capacitors included in the second capacitance element group.
 5. An impedance matching method achieving impedance matching between a high-frequency power source and a load by a variable capacitor provided between the high-frequency power source and the load, the variable capacitor including a plurality of capacitance elements respectively having a capacitor and a semiconductor switch, the method comprising: calculating a reflection coefficient based on matching information obtained by an impedance information detection unit concerning an impedance viewed from the high-frequency power source toward a load side of the load; updating on or off states of the semiconductor switches included in the plurality of capacitance elements with a first update cycle where the reflection coefficient is equal to or more than a predetermined value; and updating the on or off states of the semiconductor switches included in the plurality of capacitance elements with a second update cycle longer than the first cycle where the reflection coefficient is less than the predetermined value. 